1. Field of the Invention
The present invention relates to a method for eliminating shrinkage or porosity defects in aluminum cylinder block castings having cast-in-place metallurgically bonded cylinder liners.
2. Description of the Related Prior Art
When producing an aluminum cylinder block for an automotive engine, it is often necessary to include cylinder liners made of high wear resistant materials in order to increase longevity of the cylinder block.
There are a number of different ways of securing the liners in the block. One way is to metallurgically bond the liners to the block. In this process, the liners are typically coated with a low melting point metal material, such as zinc-based metal, which is compatible with both the aluminum cylinder block material and the selected material (e.g., cast iron liners), such that the zinc coating is capable of alloying with each of these materials.
After the liners have been coated, they are disposed in a casting cavity of a cylinder block mold and molten aluminum is cast into the cavity and around the liners. The molten aluminum melts the coating on the liner and the coating then alloys with the liner and cylinder block materials. When the casting solidifies, the liner is thereby metallurgically bonded to the resultant cylinder block. Such a process is disclosed in the related U.S. Pat. Nos. 3,165,983, granted Jan. 19, 1965 and 3,276,082, granted Oct. 4, 1966, both in the name of Thomas.
In the case of a zinc-based coating, an outer surface layer develops which consists essentially of pure zinc material. On the one hand, this layer is beneficial since it provides a certain corrosion protection to the coated cylinder liners prior to casting. On the other hand, this layer can prove to be deleterious to the integrity of the finished casting since it produces porosity defects within the cylinder block during solidification of the casting. In particular, this outer layer melts during casting and infiltrates the aluminum cylinder block material. This phenomenon is particularly prevalent when casting the V-type cylinder blocks, where the liners inclined in the mold during casting since when the coating melts during casting, the dense essentially pure Zn layer of the coating is caused to pool along the downward most portion of bottom edge of the cylinder liners. When enough of this material collects along the edge, it drips off the liner and enters into the aluminum cylinder block material. This drip of coating material is denser than the aluminum cylinder block material and continues to journey into the cylinder block material.
Since the essentially pure zinc material also has a higher shrinkage rate than the surrounding aluminum cylinder block material, the progressing "drip" leaves behind a "worm-like" void or shrinkage defect within the resultant cylinder block. Such porosity defects are, of course, undesirable in high quality cylinder block castings.